There have existed for some time piezo-based aerosol generators for use in the generation of aerosolized liquids, for example, in the construction of nebulisers for aerosolization of pharmaceutical solutions for therapeutic use by delivery through aerosol inhalation to the lungs.
Existing electronic piezo based aerosol generator devices typically comprise an actuator unit which generally utilises a support substrate (generally a disc-shaped metal, such as stainless steel or a brazing metal or alloy) having a central orifice adapted to accommodate a nozzle plate, which typically has a dome shaped central portion surrounded by an edge which is suitable for mounting the plate onto the support substrate. The nozzle plate is provided with a plurality of orifices of about 3-5 microns in diameter. A ring of actuatable material, typically a piezoelectric (“piezo”) ceramic material, is mounted onto the support substrate about the central orifice. Generally, the nozzle plate is in the form of a vibratable member and typically comprises a thin flexible membrane material surrounded by a mounting flange or edge. International Publication No. WO 2009/042187 describes a typical example of such a device. Radial oscillations or vibrations generated within the piezoceramic material are transferred through the support substrate to the flexible membrane and induce vertical vibration/flexing of the nozzle plate. It is the vertical vibrating and/or flexing action of the membrane of the nozzle plate which aerosolizes a liquid by driving the liquid through the apertures in the nozzle plate by way of the vertical flexing action. A dome shaped nozzle plate amplifies the effect of the vibration. The vertical vibration/flexing of the nozzle plate membrane produces a micropumping action at the surface of nozzle plate in contact with liquid to force liquid through the plurality of orifices in the nozzle plate membrane, thereby generating an aerosolized fluid, which typically takes the form of, for example, liquid droplets, such as those of a dissolved drug, suspended in air.
In addition to holding the piezo ceramic disc and the nozzle plate, the support substrate amplifies and transmits the oscillations/vibrations generated in the piezo ceramic to the flexible membrane nozzle plate. The orifices in the nozzle plate may be funnel shaped to maximise pumping efficiency and aerosol formation. The support also acts as a base structure to isolate the piezo ceramic body from the liquid path by providing a foundation for sealant materials such as epoxy and silicone, which are typically used to isolate the piezo ceramic body from liquid and aerosol. Although the use of a support substrate has associated advantages, one drawback is that use of such substrate requires additional manufacturing processing steps and requires use of special process steps such as brazing and use of special materials such as conductive adhesives and a sealant is typically required to prevent shorting. All of these steps add to manufacturing costs and complexity. Corrosion between the support substrate and nozzle plate and/or piezo ceramic body can be another problem which can occur where moisture ingress and can lead to premature failure of the actuator.
Prior art piezo ceramic materials suitable for actuators typically comprise the piezo ceramic material which is coated with a layer of electrical contact material, typically in the form of a (thin) layer or film of a conductive material which is deposited or coated on top of the ceramic surface, in the form of a metallized electrode. For example, a film of silver electrode can be coated on the surface area of at least one face of the piezo surface. Generally, this film of electrical contact material is provided on at least one of the entire opposing surfaces of the piezo carried out during manufacture, for example, by a screen printing or sputtering process. The film of conductive material making up the electrical contact material is generally in the order of 3 to 10 micron thickness. It typically covers at least one entire side of the piezo material. The film is needed to facilitate passage of current across/through the piezo material over all or a large proportion of its surface area to cause it to vibrate. In other words, the film conductive material making up the electrical contact material functions as an electrode to assist in power transfer through the body of the piezo ceramic material. A number of prior art devices utilise distinct regions or areas of screen printed conductive material making up the electrical contact material electrodes on the ceramic body to form a drive electrode and a sense electrode which monitors and ensures resonant vibration of the body is maintained. Where present, the support substrate (typically a metal support ring) may act as an electrode for the (opposite face of the) piezo ceramic body, thereby providing a completed circuit which facilitates application of an electrical signal to be applied across the piezo for inducing vibration thereof. Generally, the piezo will be bonded to the support substrate in a manner that does not hinder electrical conduction, for example, brazing or through use of a conducting adhesive. If the metal supporting substrate is to act as an electrode, it is necessary to have the supporting substrate conductively attached (for example, bonded by a conductive epoxy material) to the piezo to ensure current can flow from the electrode through the piezo. The entire actuator can then be coated with sealant materials to resist moisture ingress. Such sealants can however result in undesirable dampening of vibration or may lead to inconsistent actuation from device to device.
Three factors affecting the lifetime of piezo ceramic actuators are humidity, operating voltage and temperature. In particular, the piezo ceramic materials used in piezo ceramic actuator are moisture sensitive. Ingressing moisture and the electric field applied can cause electrochemical transport processes in the piezo ceramic actuator, which are accelerated by higher temperatures. While it is straightforward to develop a waterproof piezo ceramic actuator utilising waterproof coatings, making them vapour tight is more difficult. When exposed to moisture over time, the piezo ceramic actuator frequently short circuits. For example a short circuit can occur between the electrodes or screen printed surface films of conducting material, which can cause irreparable damage to the piezo ceramic actuator and leads to premature device failure. To minimise this problem, prior art piezo ceramic actuator devices are often coated in a sealant material (for example, an epoxy coating) that is cured to protect the piezo ceramic actuator circuit from moisture. The entire circuit may then be encased in a protective silicon layer to increase insulation. However, water vapour can still penetrate these polymers and so the piezo ceramic actuator generally needs to be sealed from moisture ingress. Sealing increases the manufacturing costs of the piezo ceramic actuator unit. Furthermore, the attachment area where the nozzle plate is attached to the supporting substrate is particularly susceptible to corrosion and thus leaking, making the device more prone to shorting.
Overall, the prior art designs are complex and several processing steps are required to produce the device making manufacture more costly. The corrosion issues and tendency towards circuit shorting are significant problems and limit the lifetime of the device. Furthermore, since in arrangements utilising piezo materials mounted onto supporting substrates, the piezo is bonded to the support substrate, the natural vibration of the piezo device may be dampened. The resulting dampening of the actuation of the piezo ceramic actuator thus reduces the efficiency of the device, and requires supply of higher power to compensate, risking premature failure through burn out and increasing risk of moisture induced shorting.
In piezo ceramic-based aerosol generators, flow rate is controlled by voltage applied across the piezo ceramic body. For smaller piezo ceramic actuator devices, the maximum operating voltage may be limited by the piezo ceramic actuators dimensions. The optimum operating voltage must be selected carefully to achieve optimum performance requirements as too high a voltage may damage the device. Devices therefore that give higher flow rates using lower operating voltages are desirable as the lifetime of the device will be extended when compared to piezos ceramic actuators requiring higher operating voltages to achieve the same flow rate. Therefore, dampening should be avoided as far as possible.
Thus, both the construction and the energy usage of existing piezo ceramic actuator devices could be improved upon.
U.S. Pat. No. 5,823,428 and International Publication No. WO 93/10910 each describes an apparatus for atomizing a liquid in which the liquid is passed through tapered perforations in a vibrating membrane. In the embodiments described, the perforate vibrating member and a piezoelectrical annulus are bonded onto an apertured support substrate which is an annulus of a nickel iron alloy. The perforate vibrating member is bonded across the aperture of the support substrate, not directly bonded across the aperture of the piezoelectrical annulus. Use of such an apertured support substrate is typical of prior art atomiser or aerosol generators assemblies which require multi manufacturing steps to attach the piezo to the support substrate and seal the arrangement to render it impermeable to liquids. As discussed above, this type of arrangement risks dampening of the piezo actuation, corrosion of the metal substrate and short circuiting from moisture ingress between the piezo and substrate join areas. FIG. 4 shows a plan view of the unit where two metallisation electrodes can be found, the first being a drive electrode and the second an optional sense electrode. The specification discloses that these electrodes are electrically insulation through a 0.5 mm air gap between the electrode regions. The electrodes operate together to maintain resonant vibration of the actuator.
United States Publication No. US 2008/0308096 describes an aerosol generating device which includes a membrane for atomizing a liquid; an actuating device comprising a flexible substrate plate onto which a piezo actuating device is mounted. A membrane for aerosol generation is mounted onto the piezo actuating device within an aperture provided therein. The preferred flexible substrate plate material is described as flexible printed circuit boards or conductor boards onto which electrical conductors made of Cu, Ag, Al etc. are applied or boards in which such conductors are already integrated. The electrical conductors supply power to the piezo actuating device. It also describes how the actuating device is bonded or soldered onto the membrane and flexible substrate. In a number of the drawings indicate that the membrane has collar of variable width so that the actuating device can be completed or partially covered by the collar. In a preferred embodiment, it is taught to bond the membrane and the actuating device to a metal substrate which ensures occurrence of flexural oscillations.
International Publication No. WO 00/33972 describes an electrically switchable spray generator for simultaneously generating multiple streams of droplets. The device comprises a flexural plate onto which a plurality of actuator means are bonded such that the confluence of waves generated by vibration induced in the plate coincides with a linear array of nozzles. In one embodiment, the actuator means are said to be electroded by screen printed silver to within 0.5 mm of the top and bottom edges of the piezoelectric to enable electrical connection.
United States Publication No. US 2006/0207591 describes an aerosol generating assembly for inhalation therapy devices, in which an oscillatable assembly, consisting of a membrane, a piezo electric oscillation generator and a substrate to which the membrane is attached, is mounted in an encapsulating means such that at least the membrane is exposed for the supply of liquid and the generation of an aerosol, whereas the remaining parts of the oscillatable assembly are protected. Mounting occurs by means of a flexible passage which touches the assembly in the region of an oscillation nodal line, such that the oscillatory motions of the oscillatable assembly are not negatively affected. However, it is likely that the flexible nature of the passage may lead to problems arising from moisture ingress on vibration.